The present invention relates in general to a semiconductor apparatus wherein lateral high-voltage devices of 500V or higher-voltage class and an IC for controlling these devices are integrated in the same silicon chip, and in particular to a protective film formed in a voltage withstanding region of the semiconductor apparatus.
Power supply switching IC, or the like, is exclusively used for controlling individual high-voltage devices, and such a control IC is generally connected as an external circuit to the high-voltage devices. If this control IC and the high-voltage devices can be integrated in the same silicon chip, the number of components is reduced, and the resulting power supply system may be simplified.
To integrate the high-voltage devices and control IC in the same silicon chip, there is a need to prevent the control IC from operating improperly due to noises occurring upon switching. To this end, the high-voltage devices are configured to provide lateral structures, and all of their electrodes are taken out from one of opposite surfaces of a semiconductor substrate, while the other surface of the substrate is grounded so that the high-voltage devices and control IC integrated in the semiconductor substrate are connected to a common ground. In the lateral structure, however, a high voltage is applied between electrodes formed on the semiconductor substrate, in the lateral direction of the substrate, and therefore an electric field produced by the applied voltage may concentrate at local portions between the electrodes if foreign matters, or the like, are deposited on the surface of the semiconductor substrate. Such concentration of the electric field may cause reduction of the breakdown voltage of the device. Thus, the surface condition of the semiconductor substrate is extremely important in this type of semiconductor apparatus.
Conventionally, a silicon nitride film having a refractive index of 2.8 or higher, which serves as a conductive protective films is used in order to protect the surface of the semiconductor apparatus against contamination, electrically stabilize the apparatus, and thereby prevent concentration of the electric field.
In a conventional process, however, a thin silicon oxide film 11 is undesirably formed on the surface of the silicon nitride film 12, as shown in FIG. 6, and charges are accumulated at the interface between this unnecessary thin silicon oxide film 11 and the silicon nitride film 12. As a result, the silicon nitride film 12 has nonuniform or varying resistance over its area, and the electric field concentrates at local areas of this film, resulting in reduced breakdown voltage of the semiconductor apparatus.
If the semiconductor apparatus is operated in a high-temperature, high-humidity atmosphere after the silicon chip is packaged in a mold resin, oxide films 13 are formed in local portions of the surface of the silicon nitride film 12, as shown in FIG. 7, and cause reduction of the breakdown voltage. Such oxide films 13 are formed because the silicon nitride film 12, which is a conductive protective film, is more hydrophilic and more likely to beoxidized than a conventional insulating protective film. (FIG. 8 shows the chemical reaction that occurs in this silicon nitride film 12 (refer to semiconductor world 1984, 12 p 168).
The present invention has been developed so as to solve the above problems. It is therefore an object of the present invention to provide a semiconductor apparatus that does not suffer from reduction of its breakdown voltage, and a method for manufacturing such a semiconductor apparatus.
The object may be accomplished according to the first aspect of the present invention, which provides an apparatus comprising a lateral high-voltage semiconductor device which comprises: a silicon substrate; a pair of main electrodes formed on the silicon substrate; a silicon oxide film formed on the silicon substrate, such that at least a part of the silicon oxide film is located between the pair of main electrodes; and a voltage withstanding structure formed on the silicon oxide film to cover the above-indicated at least a part of the silicon oxide film between the main electrodes, the voltage withstanding structure comprising a first silicon nitride film having a refractive index of not lower than 2.8, and a second silicon nitride film formed on the first silicon nitride film and having a refractive index of not higher than 2.2.
In the semiconductor apparatus constructed as described above, the second silicon nitride film is formed as a protective layer for the first silicon nitride film, so as to prevent oxidation of the first silicon nitride film.
The above object may be also accomplished according to the second aspect of the present invention, which provides a method for manufacturing a semiconductor apparatus comprising a lateral high-voltage semiconductor device which comprises a silicon substrate, a pair of electrodes formed on the silicon substrate, and a silicon oxide film formed on the silicon substrate such that at least a part of the silicon oxide film is located between the pair of main electrodes, comprising the steps of: forming a first silicon nitride film having a refractive index of not smaller than 2.8 on the silicon oxide film to cover the above-indicated at least a part of the silicon oxide film between the main electrodes; and removing an unnecessary silicon oxide film formed on the first silicon nitride film.
In the method as described above, the unnecessary, thin oxide film formed on the surface of the first silicon nitride film is removed, thereby to prevent charges from being generated on the first silicon nitride film.
After the above-described step of removing the unnecessary silicon oxide film, a surface of the first silicon nitride film may be treated with hexamethyldisilazane.
In this manner, moisture or water molecules are surely prevented from being deposited on the surface of the first silicon nitride film.